1. Field of the Invention
The present invention relates to apparatus for measuring micro-patterns, especially for detecting and measuring the boundaries in micro-patterns.
2. Description of the Prior Art
In the prior art there is known apparatus for measuring the width of a line using a laser beam, an example of which is schematically shown in FIG. 4. In this figure, reference numeral 1 denotes a beam of laser light. Through an objective lens 2, the laser beam is focused on a test object 5 and forms a laser spot to scan the test object. When the laser spot is scattered by an edge or the like on the test object 5, the scattered light 3 is received by photo sensors or receptors 4 arranged near the objective lens 2 as illustrated by FIG. 4. The measuring apparatus detects and determines the position of the edge from the amount of relative movement between the laser spot and the test object and also from the signal coming from the photo sensors 4. FIG. 5 shows an example of the relation between the position of edges (5a, 5b) of the test object 5 and the output signal (4a, 4b) of the photo sensor 4 derived from the light scattered by the edges.
Although the prior art apparatus as shown above is able to detect and measure very small micro-patterns, the accuracy of measurement is not so high and it is unsatisfactory. The reason for this is that since the apparatus is designed to detect scattered light, there a high possibility of the entrance of stray light in the photo sensors in addition to the laser light. Therefore, there are generated noise signals which reduce the accuracy of measurement. A known method for minimizing such noise is to additionally provide an interference filter directly before the photo sensor which transmits the laser light only. However, since the spectral characteristics of the interference filter are generally variable depending on the angle of incidence, it has been found difficult to sufficiently eliminate the stray light by this solution. As another solution it has been proposed to carry out the measurement while minutely oscillating the laser beam to minimize the effect of noise on the measuring signal. But, this solution also has a drawback in that it needs a large and complicated apparatus.